sys: Add GPIO_INTR_EDGE_MASK define

This is the GPIO_INTR_EDGE_* flags, just as a convenience to let code
test flags for interrupt edginess.

Approved by: re (kib)
Sponsored by: Amazon

(cherry picked from commit f6197f2bc05b5a8df260cbf55283fddf7c76fec4)
(cherry picked from commit bebff69739ed5d9c6bbc7796a981da867420f77e)